1. Field of the Invention
The present invention relates to a rangefinder that has: a light source for outputting a measuring beam; an AM modulation unit for AM modulating the measuring beam with a modulation signal consisting of a sinusoidal wave; a light receiving unit for detecting a reflected beam that is part of the measuring beam modulated by the AM modulation unit and reflected by a measurement object; a phase difference detection unit for finding a phase difference between the measuring beam and the reflected beam; and a distance arithmetic unit for calculating a distance to the measurement object based on the phase difference detected by the phase difference detection unit.
2. Description of the Related Art
As shown in FIGS. 9A and 9B, in a laser rangefinder for measuring a distance by modulating a measuring beam outputted from a laser light source LD, irradiating it onto an object X, and detecting the reflected beam from the object X with a photodetector PD, as modulation methods of the measuring beam, two kinds of methods of the AM (Amplitude Modulation) method and the TOF (Time of Flight) method have been put into practical use. The AM method is a method where, as shown in FIG. 9A and Formula 1, the measuring beam being AM modulated with a sinusoidal wave and its reflected beam are subjected to photoelectric exchange, and the phase difference Δφ between these signals is calculated, and a distance is calculated from the phase difference Δφ; the TOF method is a method where, as shown in FIG. 9B and Formula 2, the measuring beam modulated to be pulses and its reflected beam are subjected to photoelectric exchange, and a distance is calculated from a delay time Δt between these signals. Here, L designates a distance to the object, C the velocity of light, f a modulation frequency, Δφ the phase difference, and Δt the delay time.L=Δφ·C/(4π·f)  [Formula 1]L=Δt·C/2  [Formula 2]
Rangefinders of this kind are used for a vision sensor of a robot and an unmanned transfer vehicle, an open/shut sensor of a door, a monitor sensor for detecting existence/absence of an intruder into a monitoring area, further a safety sensor that detects a human or an object approaching a danger apparatus and stops it safely, and the like. Moreover, such a rangefinder is used in the case of recognizing a shape of a vehicle and a shape of a human. For example, in the ETC system, it is used as a sensor for determining a type of a vehicle and counting the number of passing vehicles and as a monitoring sensor for detecting crowdedness and a flow of people by counting the number of people.
Although the AM method has a characteristic that multiple measuring points for phase difference calculation can be set up and accordingly averaging processing of distance calculation is easy, which results in a higher measurement accuracy, it presents a problem that it is impossible to measure accurate distance when the phase delay exceeds one period of the modulation frequency.
In order to cope with this problem, U.S. Pat. No. 5,180,992 discloses a technique whereby the measuring beam is modulated by mutually different three kinds of modulation frequencies and a correct distance is determined from the calculated distance values based on reflected beams corresponding to the respective frequencies. In this case, the accuracy of distance measurement is governed by a signal having the highest frequency.
On the other hand, in the TOF method, since a laser beam with a short emission pulse width is used, it has a feature that emission intensity can be set high compared with the AM method and accordingly detection sensitivity is high; therefore, it becomes possible to measure a long distance while satisfying Class 1 of International Standard for the Safety of Laser Products IEC (International Electrotechnical Commission) 60825-1.
Since in the TOF method a measuring point is only a rising edge point of the signal, it has a problem that detection accuracy is low compared with the AM method. In order to cope with this problem, U.S. Pat. No. 5,455,669 discloses a technique whereby the reflection signal is measured by using a rising edge signal and a falling edge signal of the counter of a 1.5 GHz clock signal for measurement, and thereby a distance is measured with a resolution of 3 GHz.
However, in the case where a distance is measured by the AM method in normal environment, sometimes movement of the object or scanning of the measuring beam causes abrupt variation in intensity of the reflected beam, degrading a signal waveform of the reflection signal that is subjected to photoelectric exchange by the photodetector, which generates a wave position error of shifting the measured phase by 360° or more.
Moreover, the U.S. Pat. No. 5,180,992 also poses a problem that it is necessary to irradiate measuring beams modulated with three kinds of frequencies to measure one point, which lengthens emission duration, and accordingly it is difficult to satisfy a regulation of Class 1 of Safety Standard IEC60825-1, which is intended to assure the safety of a laser beam required by the laser rangefinder adapted for general environment to the human eye.
Therefore, in practical, the AM method is used being limited to a rangefinder of a red laser that satisfies Class 2 of Safety Standard IEC60825-1 in limited environments, such as a case where a detection object made up of a retroreflector etc. is disposed in a predetermined movement area and is used to detect a travelling locus of crane equipment moving on a predetermined route.
However, in the case according to the U.S. Pat. No. 5,455,669, when performing a measurement even at 3 GHz by the TOF method, the measurement will be done only with the accuracy of a unit of 50 mm that is a distance converted from the frequency, and it is difficult to realize high-accuracy rangefinding in the order of detection accuracy 10 mm that is required in mounting a rangefinder on a robot etc. Moreover, there is also a problem that it is difficult to make a signal processing circuit for a GHz band in an ASIC (Application Specific Integrated Circuit), and also it is difficult to fabricate it in a small size because reduction in the number of parts cannot be attained.